The panel is also expected to file a lawsuit next week to try to enforce a subpoena against former White House counsel Donald McGahn, a key Mueller witness, if he doesn't comply before then.

That suit is expected to challenge the White House's claim that former White House employees have "absolute immunity" from testifying before Congress.

House Judiciary Chairman Jerrold Nadler filed a petition in federal court today to obtain secret grand jury material underlying former special counsel Robert Mueller's report. (PA)The move comes after Mueller's testimony Wednesday to Congress saying he had not "exculpated" President Donald Trump. (AP)

The move comes after Mueller's testimony Wednesday to Congress saying he had not "exculpated" President Donald Trump.

Democrats are trying to focus public attention on Mueller's words and on his 448-page report's contents, including several episodes in which Mr Trump sought to influence the special counsel's investigation into Russia election interference.

Mueller concluded that he could not exonerate Mr Trump on obstruction of justice. He also concluded that there was no evidence of a criminal conspiracy between Mr Trump's campaign and Russia.

Mr Nadler said ahead of the court filing that the grand jury information "is critically important for our ability to examine witnesses" like McGahn and for them to investigate the president.

It's unclear what new information might be found in the grand jury transcripts. Many of the high-profile witnesses connected to the White House, for instance, appeared for voluntary questioning before Mueller's team rather than before the grand jury.

Around 100 House Democrats have said they favour starting the impeachment process, but House Speaker Nancy Pelosi has said she wants to build the strongest case possible before making that decision, including by going to court to force witnesses to comply. (AP)

The panel has struggled to bring in witnesses like McGahn who spoke extensively to Mueller because the White House has directed them to refuse to testify. Mr Trump has said he will fight "all of the subpoenas".

Mr Nadler said the committee is still negotiating with McGahn for documents and testimony, and the committee will file the lawsuit "in very short order" if he does not comply.

The committee's court battles are beginning as the House leaves for a six-week recess and Democrats are debating whether to impeach the Republican president.

Around 100 House Democrats have said they favour starting the impeachment process, but House Speaker Nancy Pelosi has said she wants to build the strongest case possible before making that decision, including by going to court to force witnesses to comply.

The Judiciary committee's filing says the panel needs the information in order to determine whether to recommend articles of impeachment, partly an attempt to give the request more weight in the eyes of the court.

Democrats are trying to focus public attention on Mueller's words and on his 448-page report's contents, including several episodes in which Mr Trump sought to influence the special counsel's investigation into Russia election interference. (AP)

"To meaningfully consider whether to exercise this authority – as well as to exercise its other pressing legislative and oversight responsibilities – the Committee must obtain evidence and testimony in a timely manner," the filing reads.